Most of today's solar photovoltaic (PV) power sources are utility connected. About 75% of these installations are residential rooftop systems with less than 2 kW capability. These systems typically comprise a number of PV modules arranged in series configuration to supply a power converter, commonly called an inverter, which changes the direct current (DC) from the modules to alternating current (AC) to match the local electrical utility supply.
There is a difficulty with small solar power systems on residential rooftops. Gables and multiple roof angles make it difficult on some houses to obtain enough area having the same exposure angle to the sun for a system of 2 kW. A similar problem arises where trees or gables shadow one portion of an array, but not another. In these cases the DC output of the series string of modules is reduced to the lowest current available from any cell in the entire string. This occurs because the PV array is a constant current source unlike the electric utility, which is a constant voltage source.
An inverter that economically links each PV module to the utility grid can solve these problems as the current limitation will then exist only on the module that is shaded, or at a less efficient angle and does not spread to other fully illuminated modules. This arrangement can increase total array output by as much as two times for some configurations. Such a combination of a single module and a microinverter is referred to as a PV AC module. The AC output of the microinverter will be a constant-current AC source that permits additional units to be added in parallel.
PV AC modules now available suffer poor reliability owing to early failure of the electrolytic capacitors that are used to store the solar cell energy before it is converted to AC. The capacitor aging is a direct consequence of the high temperature inherent in rooftop installations.
The electrolytic capacitors in the power circuit perform two functions. First, the capacitors hold the output voltage of the PV modules close to the maximum power point (MPP) output despite variations in sunlight, temperature or power line conditions and second, the capacitors store energy at the input and even out the DC voltage variations at the power-line frequency that result from changing the DC to AC. These functions place an additional stress on the capacitor causing internal heating that adds to the effects of high external temperature.
The high temperature reached by PV system inverters is a natural consequence of their outdoor mounting. This requires a rainproof enclosure that complicates the heat removal process. The coincidence of maximum power throughput and losses at exactly the time of maximum heating by the sun on both the enclosure and the ambient air exacerbates the condition.
Existing inverter topologies have made the electrolytic capacitor an integral part of the inverter circuit because of the high capacitance value required to store energy from the PV module. If high capacitance is required, the only economic choice is the electrolytic capacitor. Plastic film capacitors are recognized as superior in aging characteristics, but are much more expensive for the same capacitance. Thus, a means to avoid use of electrolytic capacitors can contribute to the reliability of PV power sources.